Coil for a magnet and a method of manufacture thereof

ABSTRACT

A coil for a magnet used in magnetic resonance imaging (MRI) systems, the coil including a grooved resilient former and an electrical conductor disposed around the former, wherein the former includes a sheet of flexible material having a tubular shape.

FIELD OF THE INVENTION

The present invention relates to a coil for a magnet and to a method ofmanufacturing a coil for a magnet. More particularly, it relates to agradient coil and to a method of manufacturing a gradient coil and, inparticular to a gradient coil for use in a magnetic resonance imaging(MRI) system.

BACKGROUND OF THE INVENTION

MRI systems are used today for investigating a large variety of bodyparts. These systems are based on nuclear phenomena displayed by atomicnuclei having a non-zero magnetic moment (or “spin”). When such nucleiare placed in a static, uniform magnetic field, the nuclear spins arealigned by the magnetic field so as to be either aligned with or againstthe static magnetic field. The nuclear spins are not stationary, butprecess around an axis defined by the magnetic field The frequency atwhich the spins precess is known as the “Larmor frequency” ω₀. TheLarmor frequency is given by ω₀=γB₀ where γ is the gyromagnetic ratio ofthe nucleus and B₀ is the applied magnetic field. For a hydrogennucleus, γ=42.57 MHz/T.

When the nuclear spins are aligned in the static magnetic field B₀, itis possible to “flip” the spins by applying an alternating magneticfield B₁. In order to do this, the alternating magnetic field must be at90° to the static magnetic field and it must alternate at substantiallythe Larmor frequency. When such an alternating field B₁ is applied thespins will tend to align themselves parallel to B₁, and when thealternating field is removed the spins will relax back into the state inwhich they are aligned with the static magnetic field B₀. The alignmentof the spins with the alternating field decreases the magnetisation inthe longitudinal direction (parallel to B₀) and increases themagnetisation in the transverse plane (that is, the plane perpendicularto B₀), and the subsequent relaxation of the spins when the alternatingfield is removed produces the reverse effects. These changes in themagnetisation are detected in the MRI process, and are processed toprovide a visible display of the nuclei.

FIG. 1 at 11 shows a typical MRI system in block diagram form. Themagnet 12 provides the static magnetic field B₀. In principle, themagnet 12 could be a superconductive magnet, an electromagnet or apermanent magnet. However, a super-conducting magnet is commonly used,since readily provide a large, homogeneous static magnetic field. Themagnet 12 contains a bore 13 enabling the entry of a patient into thestatic magnetic field. A patient shown at 14 is inserted into the bore13 using bed arrangement 16 so as to be within the static magneticfield.

Radio frequency (R. F.) pulses generated by transmitter 22 and appliedthrough multiplexer 23 and radio frequency coil apparatus 24 act to tipthe aligned spins so as to have a projection, for example, in the X, Zplane; the X, Y plane or the Y, Z plane. The X, Y and Z nomenclaturerefers to the imaginary orthogonal axes shown at 221 used in describingMRI systems; where the Z axis is an axis coaxial with the axis of thebore hole. The Y axis is the vertical axis extending from the centre ofthe magnetic field and the X axis is the corresponding horizontal axisorthogonal to the other axes.

The spins, when realigning after the radio frequency pulse is removedgenerate, free induction decay (F.I.D.) signals which are received bythe radio frequency coil apparatus 24 and transmitted through themultiplexer to the receiving circuit 26. From the receiving circuit thereceived signals go through the controller 25 to an image processor 27.The image processor works in conjunction with a display memory 28 toprovide the image displayed on display monitor 29. It should be notedthat the radio frequency coil apparatus 24 can comprise separate coilsfor transmitting and receiving or the same coil apparatus 24 could beused for both transmitting and receiving the R. F. pulses.

In order to spatially resolve the MRI signal, encoding signals withinthe static magnetic field are provided by gradient coils (not shown inFIG. 1). There are typically three sets of gradient coils. X gradientcoils alter the strength of the static magnetic field along the X axis,Y gradient coils alter the strength of the static magnetic field alongthe X axis, and Z gradient coils alter the strength of the staticmagnetic field along the Z axis. The strength of the static magneticfield along other axes, such as XZ axis for example, can be changedusing two or three of the gradient coils in combination.

The X, Y and Z gradient coils are driven by X gradient driver 17, Ygradient driver 18 and Z gradient driver 19, respectively. It ispossible to modify the static magnetic field B₀ using the gradient coilsso that only nuclei within a small volume element of the of the patienthave a Larmor frequency equal to the frequency of the R. F. field B₁.This means that the F.I.D. signal comes only from nuclei within thatvolume element of the patient. In practice the gradient coils aresupplied with time-varying electrical currents from a power supply, suchas a power amplifier, so that the volume element in which the nucleihave a Larmor frequency equal to the frequency of the applied R.F. fieldis scanned over the patient so as to build up a 2-D or 3-D image of thepatient.

A typical prior art set of gradient coils is disclosed in, for example,“Foundations of Medical Imaging” by Z.H. Cho et al (published by WileyInternational), and is shown schematically in FIG. 2. The X gradientcoils are shown in FIG. 2(a). FIGS. 2(b) and 2(c) show the Y gradientcoils and the Z gradient coils respectively.

It will be noted that the X gradient coils and the Y gradient coilsshown in FIGS. 2(a) and 2(b) are in the form of saddle coils. In eachcase, two saddle coils are placed either side of the Z=0 plane.

In the prior art, the gradient coils are constructed over a tubularbase, the Z gradient coils are placed over the X gradient coils, andfinally the Y gradient coils are placed over the Z gradient coils(although the order in which the gradient coils are provided on theformer is not limited to this particular order).

As shown in FIG. 2(c), the Z gradient coils are axial coils ofsolenoidal form. These axial coils are wound into pre-formed slots in aninsulating former. The former is prepared by machining grooves in afiberglass tube. The tube is then split into four sections, which areglued to the tope of the X gradient coils. The Z gradient coils are thenwound in grooves 41 in the preferably fiberglass former using copperwire or copper bar 42. FIG. 3(a) is a side view of such a prior artformer 40 showing the grooves 41, and FIG. 3(b) is an end view of theformer of FIG. 3(a).

To reduce the noise created by the Z gradient coils when they areenergized, grooves 41 in former 40 are lined with a rubber sheet 43, asshown in FIG. 4, which is an enlarged partial sectional view of theformer of FIG. 3(a). Rubber sheet 43 acts as a shock absorber and dampsthe vibration of the Z gradient coils thereby reducing the maximumacceleration of the coils and the noise generated by the coils in use(the vibrations are caused by the coils moving as a result of themagnetic forces acting on the coils).

It can therefore be seen that the prior art method of constructing the Zgradient coils is expensive. In particular, the construction of finelymachined thin-walled glass-fiber tubes further cut into four sections isvery costly. Moreover, two winding steps are required after the formerhas been glued in position. Firstly the grooves are lined with therubber sheet 43 and then, secondly, the copper wire or bar is wound intothe rubber-lined grooves.

SUMMARY OF THE INVENTION

A first aspect of some preferred embodiments of the present inventionprovides a method of manufacturing a coil for a magnet comprising: acurved former; and disposing an electrical conductor around the former;wherein the step of providing the curved former comprises the steps of:

(a) manufacturing the former in a flat or substantially flat shape, and

(b) bending the former into a curved shape.

This method of the invention allows the former to be made in one pieceand then bent into position around the underlying components. Thus, theformer can be made of flat molded elements, and these are much cheaperthen machined ones.

In a preferred embodiment the former is manufactured from a resilientmaterial. This eliminates the need to dispose a rubber layer on theformer before winding the conductor.

A second aspect of some preferred embodiments of the present inventionprovides a coil for a magnet, the coil comprising a resilient former andan electrical conductor disposed around the former. Use of a resilientformer eliminates the need to provide a layer of rubber between theformer and the conductor.

In a preferred embodiment the former is made of an elastomeric material.

The present invention also provides, in a preferred embodiment thereof,an MRI system comprising a coil as defined above. In a preferredembodiment of the invention, the magnet constitutes a gradient coil ofthe MRI system.

The present invention also provides, in a preferred embodiment thereof,a method of manufacturing a coil for a magnet comprising the step ofdisposing an electrical conductor around a resilient former.

There is thus provided, in accordance with a preferred embodiment of theinvention, a method of manufacturing a coil for a magnet comprising:

providing a curved former; and

disposing an electrical conductor around the former;

wherein the step of providing the curved former comprises manufacturingthe former of a grooved resilient material.

Preferably, providing the curved former comprises providing the formerin a flat or substantially flat shape and bending the former into acurved shape.

There is further provided, in accordance with a preferred embodiment ofthe invention, a method of manufacturing a coil for a magnet comprising:

providing a curved former, and

disposing an electrical conductor around the former,

wherein providing the curved former comprises:

manufacturing the former in a flat or substantially flat shape; and

bending the former into a curved shape.

Preferably, the former comprises a groove and the electrical conductoris disposed in the groove.

Preferably, the flat or substantially flat former is formed by molding.

Preferably, the method includes securing the former after the step ofbending the former.

Preferably, the former is manufactured from a flexible material.Preferably, the former is manufactured from a resilient material.

There is further provided, in accordance with a preferred embodiment ofthe invention, a coil for a magnet, the coil comprising a resilientformer and an electrical conductor disposed around the former.

Preferably, the former is made of an elastomeric material.

Preferably, a groove is defined in the former and the electricalconductor is disposed in the groove.

Preferably, the former is tubular.

There is further provided, in accordance with a preferred embodiment ofthe invention, an MRI system comprising a coil according to theinvention, the coil constituting a gradient coil of the MRI system.

There is further provided, in accordance with a preferred embodiment ofthe invention, a method of manufacturing a coil for a magnet comprisingthe step of disposing an electrical conductor around a resilient former.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred non-limiting embodiments of the present invention will now bedescribed with reference to the accompanying drawings, in which:

FIG. 1 is a schematic view of an MRI system of the prior art;

FIGS. 2A, 2B and 2C are schematic views of a prior set of X,Y and Zgradient coils;

FIG. 3A is a side view of a prior art former on which the Z gradientcoil is wound;

FIG. 3B is a partial sectional view of the former of FIG. 3A;

FIG. 4 is an enlarged partial view of FIG. 3A;

FIG. 5A is a side view of a former on which Z gradient coils are formedin accordance with a preferred embodiment of the invention;

FIG. 5B is a partial sectional view of the former of FIG. 5A; and

FIG. 6 is an enlarged partial view of FIG. 5A;

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference is made to FIG. 5A, 5B and 6. In accordance with a preferredembodiment of the present invention the former on which the Z gradientcoil is wound is constructed in one piece. It is molded as a flat former51, from a flexible, resilient material. The grooves in the former areformed during the molding process by using a suitably shaped mold.

Since former 51 is molded in a flexible material it can easily be bentinto a tubular shape around the X gradient coil or other underlyingcomponent 53, which may be made, for example of fiberglass. The moldedformer may be secured onto the underlying component by any suitablemeans—for example by gluing. Once the former has been secured in place,the Z gradient coils can be wound in the grooves in the former by anyconventional winding process.

Moreover, since the former is molded from a flexible, resilient materialit is not necessary to line the grooves in the former with elasticsheet, since the former itself will damp the vibrations of the windings.The use of an flexible, resilient former thus provides a furthersimplification of the manufacture of the coil.

FIG. 5A and 5B show side and end views of a former in accordance with apreferred embodiment of the invention, in which at least one flat formersheet 51 is wound around and glued to a mandrill 53. The former sheet isformed with grooves 54. The sheet may fit into slots formed in themandrill, as shown in the FIGS, or may be glued atop a simplecylindrical member. The dotted lines in FIG. 5A represent the mandrillsurface underlying the former sheet FIG. 6 shows a detail of the formersheet glued to the cylinder. While a gap is shown between the formersheet and the copper bar, it should be understood that since the formeris elastic, a tight fit for the copper may be provided.

The former sheet can be made from any resilient, electrically insulatingmaterial that is sufficiently flexible to be bent into a tube around theX gradient coils or other underlying component (which will typicallyhave a radius of 600-800 mm). As example of a suitable material would bean elastomeric material, for example, such as rubber or flexible epoxyresin system.

Although the former is made of a flat, flexible, resilient material in apreferred embodiment of the invention, the invention is not limited tothis. For example, it would be possible to use a former made in two ormore curved parts, as in the conventional method, but manufactured froma resilient material. Using such a former would allow the step of liningthe grooved with elastic to be eliminated thus simplifying themanufacture of the magnet to some extent.

Conversely, the former could be molded as a flat former, but using amaterial which was not sufficiently resilient to absorb all thevibrations of the electrical conductors.

Once this former had been bent around the X gradient coil or otherunderlying component and secured in place it would be necessary to linethe former with rubber before winding the electrical conductor. Whilethis would not achieve the full advantage of using a flexible, resilientformer it would nevertheless have some advantage over the conventionalmethod, since it would eliminate the steps of dividing the former intofour parts and positioning them individually on the X gradient coil orother underlying component.

Although the present invention has been described with reference to theZ gradient coils of a magnet for an MRI system, the invention is notlimited to this. On the contrary, the present invention can be appliedto any magnetic device which comprises an electrical conductor woundaround a former such as a superconductive or resistive coil of an MRImagnet for producing the principal (B₀) magnetic field.

As used herein the terms “comprise”, “include”, have” or theirconjunctions mean “including but not necessarily limited to.”

What is claimed is:
 1. A coil for a magnet used in magnetic resonanceimaging (MRI) systems, the coil comprising a grooved resilient formerand an electrical conductor disposed around the former, wherein theformer comprises a sheet of flexible material having a tubular shape. 2.A coil as claimed in claim 1 wherein the former is made of anelastomeric material.
 3. A coil as claimed in claim 1 wherein theelectrical conductor is disposed in the groove.
 4. A coil as claimed inclaim 1 wherein the former is cylindrical.